Allow mapping over map results (#4302)

Author: schustmi

docs/book/how-to/steps-pipelines/dynamic_pipelines.md

@@ -123,7 +123,10 @@ def map_reduce():
 ```
 
 Key points:
-- `step.map(...)` fans out a step over sequence-like inputs.
+- `step.map(...)` fans out a step over sequence-like inputs. These inputs can be either
+  - a single list-like output artifact (see the code sample above)
+  - a list of output artifacts.
+  - the output of a `.map(...)` or `.product(...)` call if the respective step only returns a single output artifact
 - Steps can accept lists of artifacts directly as inputs (useful for reducers).
 - You can pass the mapped output directly to a downstream step without loading in the orchestration environment.
 

src/zenml/execution/pipeline/dynamic/runner.py

@@ -642,6 +642,15 @@ def expand_mapped_inputs(
                 "wrap your input with the `unmapped(...)` function.",
                 key,
             )
+        elif (
+            isinstance(value, Sequence)
+            and value
+            and all(isinstance(item, OutputArtifact) for item in value)
+        ):
+            # List of step output artifacts, in this case the mapping is over
+            # the items of the list
+            mapped_input_names.append(key)
+            mapped_inputs.append(tuple(value))
         elif isinstance(value, Sequence):
             logger.warning(
                 "Received sequence-like data for step input `%s`. Mapping over "